We are planning on putting a heat pump on our 24k gallon pool because we have an oversized PV solar on our roof to cover it's operating costs. We are in Southern California and we want to know how big of a natural gas heater we would need if we only needed to heat an 8x5 integrated spa. Do I need a 400,000 BTU heater to effectively heat the spa if it will already be at 85 degrees? I'd love to heat it within about a half hour or so to the upper 90s. Is this possible with a lower BTU heater and if so, what kind is best? And if we want to heat the spa in the winter, how long would it take when the ambient temperature is 53 degrees with the highs being in the high 60s and the lows being in the low 40s (I don't really know what the water temp would be)? The pool is going to be 38x16 and goes from 3.5 to 7 feet. Any advise or suggestions?
Best heater for JUST a spa?
- Thread starter Angnjam
- Start date
Welcome to TFP! 
I can't answer on the time particulars, but I think 400K is a little much for just the spa. I'm putting in a 30-32K gallon pool, and the heater is going to be a 400K BTU unit. Raypak seems to be one of the most popular heaters of all. Around here and many places I've seen on the forum tend to prefer Raypak. Take it with a grain of salt, this will be my first.
I can't answer on the time particulars, but I think 400K is a little much for just the spa. I'm putting in a 30-32K gallon pool, and the heater is going to be a 400K BTU unit. Raypak seems to be one of the most popular heaters of all. Around here and many places I've seen on the forum tend to prefer Raypak. Take it with a grain of salt, this will be my first.
Yes I understand 400,000 btu is large, but what size is appropriate to heat the spa within a half hour then? And are certain models better at certain sizes?
The bigger the heater, the faster the spa will heat up ... while using about the same amount of gas to do it.
What is the volume of your spa? If I assume 750 gallons:
750 gallons * 8.33 pounds/gallon = 6250 pounds of water
To add 40 degrees you need 6250 * 40 = 250000 BTUs
A 400k heater with 85% efficiency will add 340000 BTU/hr
So it will take 250000/340000 = ~.75 hours = ~45 minutes to heat a 750 gallon spa by 40 degrees with a 400k heater.
A 250k heater would take ~1.2 hours = ~70 minutes
So how important is that extra 25 minutes to you in the winter? Worth the added cost of the larger heater and large gas pipe required?
EDIT: These calculations do not account for heat losses to the ground around the pipes or to the air, so times are likely a little longer.
EDIT 2: Just reread your post, if you only need to add 15 degrees in the summer, then just multiple the times by 15/40 .... 17 minutes with the 400k or 26 minutes with the 250k
What is the volume of your spa? If I assume 750 gallons:
750 gallons * 8.33 pounds/gallon = 6250 pounds of water
To add 40 degrees you need 6250 * 40 = 250000 BTUs
A 400k heater with 85% efficiency will add 340000 BTU/hr
So it will take 250000/340000 = ~.75 hours = ~45 minutes to heat a 750 gallon spa by 40 degrees with a 400k heater.
A 250k heater would take ~1.2 hours = ~70 minutes
So how important is that extra 25 minutes to you in the winter? Worth the added cost of the larger heater and large gas pipe required?
EDIT: These calculations do not account for heat losses to the ground around the pipes or to the air, so times are likely a little longer.
EDIT 2: Just reread your post, if you only need to add 15 degrees in the summer, then just multiple the times by 15/40 .... 17 minutes with the 400k or 26 minutes with the 250k
Thank you jblizzle and brushpup. I know brushpup prefers Raypak, do you feel the same way jbzizzle? Or is there a brand you prefer and think is better quality?
Raypak seems to have the best rep on the forum not just by owners, but by people in the industry as well ... I have little personal experience.
Honestly, I don't prefer it, but every builder I have actually talked with prefers them. Even ones that push all the other brands really hard have
suggested the brand. Pool builders here as well as a couple of elite ones in Central TX. Then, we see the same thing on the forum a lot.
suggested the brand. Pool builders here as well as a couple of elite ones in Central TX. Then, we see the same thing on the forum a lot.
If the spa is part of the same circulation system as your pool, why not just use the heat pump to heat up the spa? Use the same calculations as Dave showed, but with the BTU output of the heat pump.
Well, I think the heat pump would be sufficient in the summer, but in the winter, I'm figuring it will take us a couple hours to get the water up 35-40 degrees for say 900 gallons of water. If the ambient temperature outside is say 58 degrees, then I am assuming to raise the water 40 degrees would take over 2 hours. I am afraid the desire to get in the spa will be gone by the time the water gets heated up.
